Purpose of this commit is to avoid errors for some performance tests that
have a slightly variable number of queries. Indeed some tests in test_mail
module vary of 1 or 2 queries due to a behavior still not found that does
not seem deterministic.
This should not be considered as a real error, but should not be considered
as completely valid. This is why a margin is accepted in some tests that
will raise a warning instead of an error. This warning tells people to
check if they have increased the query counter or if it is due to a random
effect in query count.
This is a manual forward-port of saas-11.2 commit 3fe0ee5ae6 .
Thibault Delavallée
a03f8da8e4